1. Field of the Invention
The present invention relates to semiconductor devices and, in particular, to complementary metal oxide semiconductor (CMOS) image and mass spectroscopy sensors.
2. Related Art
In the past decade, CMOS sensor arrays and in particular Active Pixel Sensor (APS) arrays for imaging including visible and non-visible applications have been consistently enhanced. Their significantly improved quality with respect to Charge Coupled Devices (CCDs) allows the sensors to be used in varied and challenging applications. A typical CMOS Active Pixel sensor array comprises a focal plane array of evenly spaced and equally sized pixel cells (pixels), where each pixel comprises a light (visible or non visible) detection component such as photodiode. Each pixel also has a readout circuit connected to its light detection node. The pixel includes at least a reset transistor, an amplifier transistor and a select transistor that connects to the column bus and may also include sample and hold circuitry for Correlated Double Sampling (CDS).
However in some extreme applications such as Laser Induced Breakdown Spectroscopy (LIBS) that require ultrafast electronic shutter speeds (referred to as gating), large dynamic range and low noise CMOS sensors have not been used.
Instead, the most widely used sensor in these applications is the Intensified Charge Coupled Device (ICCD) which provides high speed gating, high gain and 2-D imaging to support the Echelle spectrograph. A typical ICCD comprises an input photocathode followed by a micro-channel plate (MCP) electron multiplier, a phosphorescent output screen and a CCD. The gain of the micro-channel plate is adjustable over a wide range, with a typical maximum of about 80,000 photons pulse from the phosphor screen per one photon input. The multiplied photons are then sensed by the CCD. By amplifying each photoelectron by a gain as high as 100,000, the device essentially eliminates the readout noise of the CCD.
ICCD, however, suffers from several drawbacks. For example, Image Intensifier devices suffer from increase in Fixed Pattern Noise (FPN) due to the non-uniformity of the photoelectron gain of the device across the entire imaging area. The gain non-uniformity is further degraded due to the delay of the gating signal arrival between the edge and the center of the Intensifier (caused by the photocathode finite conductivity and referred to as “Irising”). That causes reduction in Signal to Noise Ratio (SNR) and Dynamic Range and increases device complexity for FPN correction functionality.
Further signal degradation is caused by the gain uncertainty for each interaction (referred to as electron multiplication noise) that manifests similarly to Shot Noise. This effect is characterized by the “noise factor” parameter (NF). A typical best case NF value is ˜1.7. The NF has the equivalent effect of lowering the native Quantum Efficiency (QE) of the device by the square of NF. Thus, the effective QE of an Image Intensifier device with a native QE of 45% and best NF of 1.7 will be reduced down to 15.571%.
An additional problem is the limited spectral response of the Image Intensifier device, which has poor sensitivity at the longer red wavelengths, UV, and deep blue, a characteristic that is often harmful to the performance of a wide spectra range imaging systems such as Laser Induce Breakdown Spectroscopy (LIBS) or other spectroscopic systems and thus not desired.
Furthermore, an Image Intensifier device suffers from relatively low intra-frame dynamic range due to the high intensifier gain which will cause the CCD to saturate earlier. For example with Intensifier gain of 100, the effective dynamic range will be reduced by 100:1; thus a CCD with full well of 100,000 photons will effectively saturate at 1000 signal photons. Typical gains used are higher and can go up to 100,000 and hence squash down the dynamic range further.
This problem is particularly damaging to the Echelle spectrograph that simultaneously provides the strongest lines of the major elements and the weak lines of the trace elements.
The intra-frame dynamic range problem is further convolved by localized dynamic range problems. The localized dynamic range is significantly decreased due to the “halo” phenomenon of the Intensifier. The halo is the bright disk formed around high intensity points (or lines) due to electro-optical effect that occurs in the photocathode-to-MCP region in the intensifier tube. Some of the photoelectrons are not captured by the MCP; rather they are reflected azimuthally in various directions back toward the photocathode and reflected back to the MCP on which they land in a circular zone around their starting point. The halo unfortunately masks any neighboring lower signals thus reducing the local dynamic range. As mentioned above, this becomes a severe issue for the Echelle spectrograph that simultaneously provides the strongest lines of the major elements and the weak lines of the trace elements.
In addition, there is further Modulation Transfer Function (MTF) degradation (can be observed as reduced sharpness, reduced contrast and reduced spatial resolution) due to the crosstalk between channels of the Micro-channel Plate (MCP). The MTF is also degraded due to the Halo phenomenon, mentioned previously, that increases the Point Spread Function (PSF).
Accordingly, it is desirable to have a CMOS sensor array that can provide fast gating, large dynamic range and QE without the disadvantages discussed above associated with Intensified CCDs.